Problem: Simplify the following expression: $y = \dfrac{1}{5n} - \dfrac{1}{9n}$
Explanation: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $5n$ and $9n$ $\lcm(5n, 9n) = 45n$ $ y = \dfrac{9}{9} \cdot \dfrac{1}{5n} - \dfrac{5}{5} \cdot \dfrac{1}{9n} $ $y = \dfrac{9}{45n} - \dfrac{5}{45n}$ $y = \dfrac{9 -5}{45n}$ $y = \dfrac{4}{45n}$